Quasi-flexural folding of pseudo-bedding

Abstract

The disharmonic outcrop-scale folds and associated structures in a 300-m-thick Cretaceous limestone formation in the Pindos Group exposed on Mount Lykaion (Arcadia, Greece) can be understood only in the context of superposed pre-tectonic and tectonic mass-transfer dissolution creep. Though the structures described may seem unusual, they are undoubtedly abundant in strongly deformed pseudo-bedded limestone formations. In this example the Thick White Limestone Beds (TWLB) formation displays pervasive flaggy pseudo-bedding, which is a diagenetic stylolitic layering resulting from pressure dissolution. The mass-transfer dissolution creep that produced the diagenetic stylolitic surfaces was achieved during gravitational loading accompanying burial, compaction, and lithification, and was driven by high stress concentrations at grain contacts. The diagenetic stylolitic surfaces are lined with insoluble residue and commonly are marked by interlocking stylolite teeth oriented perpendicular to the pseudo-bedding. At the outcrop scale, individual pseudo-beds display significant thickness variability. Moreover, the individual pseudo-beds not uncommonly wedge out and disappear laterally over very short distances. Along with all the Pindos Group strata, the TWLB formation was profoundly shortened in the Late Cretaceous and early Tertiary as a result of inversion tectonics. The Pindos Group limestone-dominated formations underlying and overlying TWLB readily accommodated flexural-slip folding, for these formations are marked by optimum mechanical stratigraphy, ubiquitous thin bedding, and pervasive bedding-plane slip surfaces. For the TWLB formation, however, strain accommodation by flexural-slip kinematics was spotty and inefficient, primarily because individual diagenetic stylolite surfaces are by no means planar or smoothly curviplanar; they tip out over short distances; and their interlocking stylolite teeth create elevated frictional resistance to layer-parallel slip. These factors, plus the apparent ease with which the limestone pseudo-beds deform by tectonically induced mass-transfer dissolution creep, were responsible for the distinctive structures observed in outcrop. These include indentation structures, where a part of one fold limb indents into the other, or where the hanging wall of a small thrust indents into its footwall, or where the limbs of a common anticline or syncline migrate toward one another by pressure dissolution removal of the axial-trace region. In terms of the overall progressive deformation, tectonically driven layer-parallel shortening of pseudo-beds first resulted in tectonic stylolites perpendicular to pseudo-bedding, with teeth parallel to pseudo-bedding. This was superseded at the mesoscopic scale by a layer-parallel shortening achieved by buckling of pseudo-beds, with loci of buckling and incipient buckling geometries influenced primarily by pseudo-bed thickness (and variations in same), wedge-outs of pseudo-beds, and the mechanical properties of the pseudo-beds. In some cases shortening and tightening of buckle folds involved flexural-slip kinematics, accommodated in part by slip or shear along clayey insoluble residue at pseudo-bed contacts. Where diagenetic stylolite surfaces abruptly tipped out, or where friction along them was excessively high, the slip deficit (and thus shortening deficit) was replaced by mass interpenetration of pseudo-beds, stylo-faulting, stylo-duplexing, and other forms of mass-transfer dissolution creep. Slip toward the tip lines of wedge-outs even may have produced end loading that, in turn, generated adjacent localized buckle folds. The properties of the folds and associated structures in the TWLB formation are introduced here as a new category of quasi-flexural folds in the Donath and Parker (1964) classification.